A. Technical Field
The present invention relates generally to adaptive equalizer training in communication systems, and more particularly, to the implementation of adaptive equalization on active, initialized links.
B. Background of the Invention
High-speed serial protocols like Serial Attached SCSI (SAS) implement adaptive equalizer training during link initialization. Channel characteristics change over time due to environmental factors such as temperature, fluctuations in power supply voltage, and unintended interactions with surrounding circuitry resulting in signal degradation. Equalizers train to remove intersymbol-interference (ISI) caused by dielectric loss, connector loss, return loss, distortions, and other channel characteristics that alter a signal from the point of transmission to the receiver. As speeds of modern communication systems increase there is a meaningful importance in achieving low bit-error ratios (BER) and maintaining signal integrity.
Traditional control algorithms do not permit an opportunity to re-optimize the equalizer after a link is established without performing a link reset and a new link initialization sequence. This reinitialization requires significant time to complete and is disruptive at the system level.
Equalizers operating in large systems with many subsystems undergo significant changes in thermal and supply voltage operating points as the system is brought from a non-operational to fully operational state. In addition, switching to power saving modes induces a change in thermal operating point. It is likely that a receiver equalized at one temperature and voltage will be non-optimal at a significantly different environmental operating point. These environmental fluctuations can alter channel characteristics after equalization coefficients have already been set, leaving the system sub-optimally or improperly equalized.
Therefore, it is desirable to be able to re-optimize the receiver coefficients on an active link when environmental changes take place for any given hardware to achieve optimal link margin without the need for a disruptive re-initialization.